This invention relates to methods for improving the adherence of intrinsically conductive organic polymers to anodic valve metal dielectric films and capacitors prepared by use of the method which exhibit reduced frequency of short failures and reduced equivalent series resistance (ESR).
Electrolytic capacitors having aluminum anode and cathode foils separated by layers of absorbent paper, wet with an liquid electrolyte, have been an item of commerce since the 1920""s, as have electrolytic capacitors having a powder metallurgy tantalum anode impregnated with a highly conductive liquid electrolyte (such as an aqueous solution of sulfuric acid or lithium chloride). There are many liquid electrolyte solutions which have been proposed and/or adopted for use in electrolytic capacitors. Some of these electrolyte solutions contain materials which render them solids or semisolids at room temperature. Mannitol, various gums, poly-vinyl-pyrrolidone, etc. have been employed in capacitor electrolyte fabrication for the purpose of increasing the sparking voltage, increasing the viscosity (to the point of solidity at room temperature) or reducing the vapor pressure of electrolyte solutions. Despite the addition of materials to liquid electrolyte solutions which tend to thicken or solidify these solutions, they remain essentially liquids and conduct current by an ionic conduction mechanism.
It was recognized fairly early in the development of electrolytic capacitors that true solid-state conductors would have to be found which were compatible with the anodic oxide films covering the valve metal anodes used in electrolytic capacitors to substantially reduce the resistance of the cathode layer and overall resistance of these devices. In 1933, J. E. Lilienfeld obtained a patent (U.S. Pat. No. 1,906,691) covering the use of true, solid-state cathode materials, such as cuprous sulfide, cupric oxide, copper/cupric oxide combinations, lead oxide, etc., covered by a layer of silver, nickel, aluminum, copper, etc., for the purpose of reducing device resistance.
Lilienfeld""s solid capacitors apparently never went into production due to the onset of the Great Depression.
A successful solid-state electrolytic capacitor was patented by Haring et. al., in 1963 (filed on Apr. 2, 1953), U.S. Pat. No. 3,093,883. This device employs pyrolytic manganese dioxide (produced via the pyrolysis of aqueous manganese nitrate solutions) as the cathode material.
In a 1967 patent (U.S. Pat. No. 3,345,544), Metcalfe extended the manganese dioxide cathode technology to include anodized aluminum foil anodes. Metcalfe""s major finding was that, in order to resist the corrosive effects of manganese nitrate pyrolysis, the aluminum anode foil required phosphate and/or chromate anodizing electrolyte solutions (it has since been determined that anodic aluminum films formed in phosphate solutions consist largely (up to 90%) of aluminum phosphate, a very insoluble and non-reactive aluminum compound). Such films are better referred to as anodic layers than oxide layers.
The equivalent series resistance (E.S.R.) and dissipation factor (d.f.) tends to be much lower for capacitors fabricated with manganese dioxide cathode material than for equivalent capacitance capacitors constructed with liquid electrolyte solutions due to the lower resistivity of manganese dioxide (1-3 orders of magnitude lower resistivity than liquid electrolyte solutions).
With the continuing development of ever-faster microprocessors and lower-voltage logic circuits, the demand for lower E.S.R. capacitors for use in conjunction with faster micro-processors has motivated capacitor manufacturers to develop solid state cathode materials which are more conductive (less electrically resistive) than manganese dioxide.
In the early 1980""s, aluminum electrolytic capacitors were introduced which were fabricated having a T.C.N.Q.-amine complex acting as the cathode. These capacitors established the stability and high conductivity achievable with solid-state organic cathode materials. The ongoing effort to increase the maximum temperature capability of organic cathode electrolytic capacitors has led to the development of methods of capacitor fabrication employing intrinsically conductive organic polymers, such as polypyrrole, polythiophenes such as polyethylene dioxythiophene, polyanilines and polyacetylenes. Numerous substituted monomers (derivatives) are useful as are mixtures of two or more monomers from different types, i.e., mixtures. These are relatively high molecular weight materials which possess electrical conductivity along a poly-acetylene-like backbone contained within the polymer and as such there is minimal interfacial resistance such as is encountered by electrons traveling through the myriad of layers of charge-transfer complexes, such as the T.C.N.Q. or T.C.N.E. charge transfer complexes. As a result of the higher molecular weight, intrinsically conductive organic polymers not only tend to possess higher conductivity than charge-transfer complexes, but also tend to exhibit higher thermal stability. While charge-transfer complexes tend to become unstable at temperatures above about 105xc2x0 C., intrinsically conductive polymers, such as polyethylene dioxythiophene, exhibit high temperature stability to 150xc2x0 C. or even 175xc2x0 C. in the absence of atmospheric oxygen.
There are two routes used to deposit intrinsically conductive organic polymers on electrolytic capacitor anode bodies:
1) Chemical
2) Electrochemical
The first of these approaches is well described in U.S. Pat. No. 4,910,645, to Jonas et. al., and consists of immersing the anodized substrate sequentially in an aqueous solution of an oxidizing agent such as, but not necessarily limited to, ammonium persulfate, and a dopant acid and/or a salt of a dopant acid, such as p-toluene sulfonic acid or sodium p-toluene sulfonate [Intrinsically conductive organic polymers generally contain one dopant acid anion for each 3 to 4 monomer units which have been joined to form the polymer. The presence of a strong dopant acid anion is thought to result in a delocalization of electric charge which provides electrical conductivity]. After a drying step, conducted to free the pores of the anode bodies from the water portion of the oxidizer/dopant solution, the anode bodies are then immersed in a solution of the monomer, usually dissolved in at least a partially organic solvent such as a low molecular weight alcohol, etc. Once the solution of monomer, which may consist entirely of monomer, is introduced into the capacitor anode bodies, the anodes are allowed to stand, generally in an oven above room temperature, to facilitate production of the intrinsically conductive polymer material. Repeat dipping sequences may be employed to more completely fill the pore structure of the anode bodies. In practice, rinsing cycles are generally employed to remove reaction by-products, such as ammonium sulfate, sulfuric acid, iron salts (when an iron (III) oxidizer is employed), or other by-products depending on the system employed. Chemical production of intrinsically conductive organic polymers may also be carried-out with capacitor anode bodies by first introducing the monomer to the capacitor bodies, followed by introduction of the oxidizer and dopant (the reverse order of polymer pre-cursor introduction described above). It is also possible to mix the dopant acid(s) with the monomer solution rather than with the oxidizer solution if this is found to be advantageous. U.S. Pat. Nos. 6,001,282 and 6,056,899 describe a chemical means of producing an intrinsically conductive organic polymer through the use of a single solution which contains both the monomer and the oxidizing agent, which has been rendered temporarily inactive via complexing with a high vapor pressure solvent. As the solution is warmed and the inhibiting solvent evaporates, the oxidative production of conductive polymers ensues. The dopant acid anion is also contained in the stabilized poly-precursor solution.
The second approach to the production of intrinsically conductive organic polymers within capacitor anode bodies is electrochemical. U.S. Pat. No. 6,324,050 describes a method of producing a solid-state electrolytic capacitor by completely coating the interstices of an anodized valve metal anode body with a continuous layer of manganese dioxide, then through an electrical contact contacting the aforesaid manganese dioxide layer, immersing the anode bodies in an electrically conductive solution of conductive polymer pre-cursor monomer and dopant acid anions, then passing current through the manganese dioxide layer (which is biased positive relative to the solution) in order to deposit a conductive polymer coating atop the continuous manganese dioxide layer. Washing steps, etc. may be carried-out following the polymer production step.
The polymer coated capacitor anode bodies, coated either chemically or electro-chemically with an intrinsically conductive organic polymer cathode layer, may then be processed into completed capacitors by coating the conductive polymer cathode coatings with graphite paste, conductive paint, etc., as is well known to those skilled in the art.
Electrical xe2x80x9cagingxe2x80x9d or xe2x80x9creformxe2x80x9d steps may be carried-out between and/or after the polymer production steps in order to improve the leakage current performance of the finished capacitors, as is well known to those skilled in the art of capacitor manufacture.
The demand for capacitors exhibiting lower equivalent series resistance and dissipation factor, which has led to the development of electrolytic capacitors containing conductive polymer cathode materials, has been accompanied by a demand for capacitors exhibiting higher reliability, particularly a lower incidence of high leakage current/short circuit failures. The quality and stability of the anodic oxide dielectric film is the most fundamental aspect of electrolytic capacitors so far as the reliability of these devices is concerned. It has long been known that the life expectancy of xe2x80x9csolidxe2x80x9d (i.e., solid-state cathode) tantalum capacitors, expressed as failure rate, is an inverse exponential function of the field existing across the dielectric during use (see, for example, the paper, entitled xe2x80x9cFailure Mechanism of Solid Tantalum Capacitorsxe2x80x9d, by B. Goudswaard F. J. J. Driesens, which appeared in xe2x80x9cElectrocomponent Science and Technologyxe2x80x9d, Vol. #, pp. 171-179, 1975).
In addition to the relationship between the oxide dielectric thickness and device reliability, the chemistry of dielectric films and dielectric film growth has been investigated extensively. It is known that anodic oxide films grown on tantalum in phosphate solutions tend to have fewer crystalline oxide flaws than with other electrolyte solutions (See, for example, xe2x80x9cHigh Field Ionic Conduction in Tantalum Anodic Oxide Films With Incorporated Phosphatexe2x80x9d, by C. J. Dell""Oca and L. Young, last paragraph. This paper appeared in xe2x80x9cThe Journal of the Electrochemical Societyxe2x80x9d, Vol. 117, No. 12, December, 1970, pp. 1548-1551.). The presence of incorporated phosphate in anodic tantalum oxide dielectric films is also known to inhibit oxygen migration through these films. The phosphate is present in the outer portions of the films (i.e. the portion produced by tantalum ion migration) and the outer, phosphate-containing region is a greater fraction of the total film thickness, contains a higher concentration of phosphate, and is more resistant to oxygen migration degradation as the concentration of phosphate in the anodizing electrolyte is increased (c.f. xe2x80x9cThe Heat-Treatment of Anodic Oxide Films on Tantalum IV. Anodization in Phosphoric Acid Solutionsxe2x80x9d by D. M. Smyth, T. B. Tripp, and G. A. Shirn, xe2x80x9cJournal of the Electrochemical Societyxe2x80x9d, Vol. 113, No. 2, February, 1966, pp. 100-104.).
Due to the observed higher film quality (i.e. fewer flaws) and higher film thermal stability observed with higher phosphate content anodic tantalum oxide dielectric films, a great deal of effort has been expended in the development of high phosphate content anodizing electrolytes, as reflected in U.S. Pat. Nos. 5,716,511, 6,162,345, and 6,480,371. The electrolyte compositions covered by these patents allow the use of relatively high (up to 10%) phosphate content solutions for high phosphate incorporation into the anodic oxide while maintaining product throughput and ease of use necessary for mass production.
As mentioned above, anodic films formed on aluminum capacitor substrates in phosphate solutions are also much more stable with respect to hydration degradation than are films formed in non-phosphate solutions (c.f. U.S. Pat. No. 3,345,544, Metcalfe). The anodizing of aluminum foil in phosphate solutions has traditionally been accompanied by a significant amount of corrosion of the foil to form aluminum phosphate particles which must be removed from the anodizing electrolyte. The degree of corrosion is particularly severe with highly etched foil and, especially higher anodizing voltages. Recently, processes have been developed which facilitate high-voltage anodizing (U.S. Pat. No. 6,409,905) and reel-to-reel anodizing (U.S. Pat. No. 6,540,900) of aluminum capacitor foil in phosphate solution with much lower amounts of foil corrosion than was found with earlier phosphate anodizing systems.
With systems available to produce high phosphate content anodic films on tantalum, aluminum and niobium for high anodic film stability, and the need for minimal E.S.R., anode bodies coated with high phosphate content anodic dielectric films have been coated by chemical oxidation processes with intrinsically conductive organic polymers and processed into finished capacitors. Unfortunately, the equivalent series resistance and leakage current for these devices was higher than anticipated and the capacitance was lower than anticipated, both suggesting a weak attachment of the conductive polymer to the anodic film substrate.
One aspect of high phosphate content anodic oxide dielectric films is that the anodic oxide becomes more acidic as the proportion of acid phosphate is increased. An example of the rapid increase in acidity with increasing phosphate content is given for bulk aluminum oxide in xe2x80x9cSolid Acids and Bases, Their Catalytic Propertiesxe2x80x9d (by Kozo Tanabe, 1970, Kodansha, Tokyo, Academic Press), on page 89, in which a doubling of the phosphate content, from 0.1 to 0.2 mmol/gram, increases the surface acid strength by 6 orders of magnitude.
Unfortunately, highly acidic surfaces do not tend to wet well with solutions of the highly acidic dopants which are used to dope intrinsically conductive organic polymers (as described in U.S. Pat. No. 4,910,645, Jonas, et. al.). We have traced at least a major portion of the excessively high leakage currents and equivalent series resistance observed with valve metal capacitor bodies anodized in phosphate and coated with chemically produced conductive polymers which have good conductivity to the absence of good attachment of the conductive polymer to the underlying anodic film surface. While not being bound by any theory, it is believed that the acidic nature of the dopant solution and the resulting conductive polymer inhibits the wetting of the acidic anodic film by the dopant and resulting polymer, thus limiting the adherence of the polymer to the anodic dielectric film.